The kraft pulping process has assumed major importance since its development approximately 100 years ago out of the earlier and simpler alkaline processes. Basically the process relies upon the efficiency of sodium hydroxide combined with sodium sulfide in breaking the intercellular lignin bonds such that wood can be separated into individual fibers. The kraft process and variants of it are useful not only in providing the vast amounts of pulp needed for paper-making, but also in supplying pulp to other users such as the viscose process to produce fiber for synthetic fabrics, and other cellulose-based chemical industries.
The exact nature of the action of sodium hydroxide and sodium sulfide on the lignin bond is not entirely understood, but the kraft process has nevertheless managed to contend very successfully against the principal competitive pulping processes using sulfite chemicals. The success of the kraft process is accountable partly to the broad spectrum of woods which it can successfully convert into pulp. In this regard it is notably superior to the sulfite processes in pulping many hardwoods, and in utilizing scrub wood, trimmings and other low grade pulp sources formerly not thought suitable for paper making.
One of the principal hindrances to the growth of the kraft process in competition with sulfite and other processes has been the necessity to utilize large quantities of expensive alkali chemicals. Long ago, the evaporation and subsequent recombustion of waste pulp processing fluids, or "liquors" was utilized as a way of recovering, re-calcining, and causticizing limestone. Today these recovery processes have been refined and made more efficient and economical by employing more sophisticated and economical lime-sludge recovery means, such as settling and filtration. These measures and many others have significantly aided in reducing cost, minimizing wastage of precious raw materials, and in other ways improving the versatility and efficiency of the kraft process.
Recently, growing public and government concern over the environmental effects of all industrial operations, but particularly those involving the disposal of quantities of chemical waste, has prompted further investigation into recovery processes and systems which might be useful in large volume chemical industries such as the pulp and paper industry. One side effect of these concerns has been that the large-scale disposal of chemical effluents from basic processes has become expensive to the industry concerned, not only because of the loss of potentially useful chemicals but also because environmental standards have made the disposal itself a costly operation.
In terms of sheer volume and expense, the principal raw starting chemical material in the kraft pulping process is limestone (impure calcium carbonate). Since around 250 kilograms of lime for each tone of pulp are needed in the process, it is obvious that the availability of limestone and the cost of processing it and disposing of any waste are extremely significant in a large kraft pulping industry. As already mentioned, pulping mills long ago began to recover lime sludge from their waste processing liquors by evaporation, settling or filtration such that the considerable calcium carbonate content of the lime sludge could be re-calcined to derive calcium oxide, which could then be converted to calcium hydroxide in the slaking operation. While these measures have achieved a great cost and raw material saving and by now must be considered virtually essential, little has yet been done to recover the lime and associated chemicals which are lost in the form of the waste from the slaker, commonly referred to as "limestone grits".
Limestone grits may be considered as whatever portion of the kiln-burned limestone charged into the slaker proves to be unreactive and is rejected as waste. As already noted, the limestone starting material is mostly calcium carbonate (approximately 90 percent or more) with the balance being comprised of compounds of silicon, iron, magnesium, chromium and possibly other trace constituents. At the high temperatures involved in the calcining operation, the silicon compounds can actually fuse and coat the limestone particles, rendering them unreactive in the following slaking operation. They will then form one constituent of the limestone grits. Another and more important component of limestone grits are particles of under-calcined or over-calcined limestone. The percentage of these present in limestone grits varies according to the efficiency of operation of the kiln used in the calcining operation.
Under-calcined limestone consists of particles in which at least some portion of the calcium carbonate has not been converted to calcium oxide. Although it is not immediately useful in the kraft pulping process, under-calcium limestone can be recovered in the lime sludge and be successfully re-calcined.
Over-calcined limestone is fully converted to calcium oxide but due to faulty calcining has a dense closely packed molecular structure which does not slake readily. Given sufficient time in contact with water, over-calcined limestone will swell and most of it will then slake.
In addition to the above constituents of limestone grits emerging from the slaker, a considerable quantity of caustic chemicals coats the various particles comprising limestone grits and is thus lost with them in the event they are simply wasted. In the past, limestone grits have not been recovered and reused principally because there seemed to be no economical method of separating the various constituents for reuse.
Without such separation the undesirable constituents in limestone grits are too numerous to permit reuse of the grits in the kraft process cycle. In particular, the silicon is especially harmful since it takes the form of silicon dioxide or silica which results in harmful kiln ring formations. Other harmful ingredients in the grits include high concentrations of metallic ions, in particular iron, which reduce the efficiency of the causticizing. Consequently, any scheme for rendering the limestone grits reusable in the kraft pulping process must either eliminate these harmful constituents or reduce their concentration to a level no greater than the level at which they are present in raw limestone.
This patent application is directed to a process and apparatus for the recovery of limestone grits by fractionation or separation into the discrete components of the grits. As will appear from the remainder of the patent application, by the process and apparatus of this invention these grits are capable of being economically fractionated into more or less usable components, such that any waste is considerably smaller in volume and is free of the caustic chemicals which formerly caused its classification as an environmentally dangerous waste product.
Fundamentally, the process and apparatus of this patent application are designed to take advantage of the discovery by the inventor that separation of the particulate portion of limestone grits into two parts based on particle size alone is adequate to recover virtually all of the limestone which has an acceptable impurity level.
Specifically, the particles of approximately 1 mm. size and smaller have an impurity concentration scarcely higher than that of raw limestone. These particles can profitably be returned to the kraft cycle. Recovery of the remaining important constituent in grits, the valuable but environmentally dangerous caustic chemicals, depends only upon establishing reasonable efficiency in washing all the particles in the limestone grits. The particles larger than 1 mm. in size can then be rejected, together with virtually all the harmful impurities, from the kraft cycle. Moreover these particles, now freed of their coating of caustic chemicals, can find beneficial uses in other industries.
By applying the above principles in a process and apparatus designed to maximize the efficiency of recovery, the ability of the kraft process to remain economically competitive while minimizing raw resource wastage and environmental hazard is promoted.